In laser technology, crystals are used for many purposes. Laser crystals are among the most common. These are crystals that can be optically pumped and then are able to amplify light of a suitable wavelength. A simple laser oscillator consists of such a laser crystal, placed between two mirrors. The two mirrors, if properly aligned, are said to form a resonator, and the optically pumped laser crystal is called the gain medium.
A second, fairly popular class of crystals are the electrooptical devices. They alter, depending on an applied electric field, the state of polarization of light passing through them. Together with polarizing devices they serve as optical modulators and ultimately as optical switches. When placed into a resonator cavity they are called Q-switches. because they switch the Q-value (Q meaning quality) of the resonator from low (oscillation inhibited) to high (oscillation enabled). There are means other than electrooptical crystals to Q-switch a laser oscillator. For example there are acoustooptic Q-switches made of TeO.sub.2, SF.sub.10 and LiNbO.sub.3.
A third class of crystals are the so-called nonlinear optical crystals (hereafter sometimes referred to as nonlinear or NLO crystals). Under favorable conditions (high intensity, phase matching) they convert light of a given wavelength into light of a different wavelength. Probably the most popular of these processes is second harmonic generation, also called frequency doubling, and in the majority of cases this means converting infrared light of 1064 nm wavelength (a popular laser wavelength) into green light of 532 nm wavelength.